<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        *{
            margin: 0;
            padding: 0;
        }
        li{
            list-style-type: none;
        }
        a{
            text-transform: none;
            font-size: 14px;
        }

        .nav{
            margin: 100px;
        }
        .nav>li{
            position: relative;
            float: left;
            width: 80px;
            height: 41px;
            text-align: center;
        }
        .nav li a{
            display: block;
            width: 100%;
            height: 100%;
            line-height: 41px;
            color: #333;
        }
        .nav>li>a:hover{
            background-color: #eee;
        }
        .nav ul{
            display: none;
            position: absolute;
            top: 41px;
            left: 0;
            width: 100%;
            border-left: 1px solid #fecc5b;
            border-right: 1px solid #fecc5b;
        }
        .nav ul li {
            border-bottom: 1px solid #fecc5b ;
        }
        .nav ul li a:hover{
            background-color: #fff5da;
        }
    </style>
</head>
<body>
   <ul class="nav">
       <li>
           <a href="#">微博</a>
           <ul >
               <li><a href="#">私信</a></li>
               <li><a href="#">私信</a></li>
               <li><a href="#">私信</a></li>
           </ul>
       </li>
       <li>
        <a href="#">微博</a>
        <ul >
            <li><a href="#">私信</a></li>
            <li><a href="#">私信</a></li>
            <li><a href="#">私信</a></li>
        </ul>
        </li>
        <li>
        <a href="#">微博</a>
        <ul >
            <li><a href="#">私信</a></li>
            <li><a href="#">私信</a></li>
            <li><a href="#">私信</a></li>
        </ul>
        </li>
   </ul>
   <script>
       var nav = document.querySelector('.nav');
       var lis = nav.children;
       console.log(lis);
        for(var i =0;i<lis.length;i++){
            lis[i].onmouseover = function(){
                this.children[1].style.display= 'block';
            }
            lis[i].onmouseout = function(){
                this.children[1].style.display = 'none';
            }
        }
    
   </script>
</body>
</html>